The present invention relates to a receiver for receiving a high definition signal, and, more particularly, to a receiver employing an NTSC rejection filter for eliminating noise and other interference from the high definition signal.
In the United States, a test has been conducted of the Grand Alliance Advanced Television (GA-ATV) system for digital television transmission. GA-ATV is a new television standard capable of replacing NTSC. The GA-ATV system (also referred to as a GA-HDTV or GA-VSB system) standardized by the Advanced Television System Committee (ATSC) employs vestigial sideband (VSB) modulation for digital transmission. For this modulation, 8-VSB using eight levels is selected for a terrestrial broadcast mode and 16-VSB using sixteen levels is selected for a high speed cable mode.
A new ATV signal is simultaneously transmitted with a conventional analog television signal (NTSC) through a television channel which is not used in a given geographic area (a so-called `taboo channel`). Accordingly, the GA-VSB receiver must be designed to be robust in the presence of NTSC co-channel interference.
Meanwhile, a comb filter is proposed by the ATSC standard as an NTSC co-channel interference rejection filter (NTSC rejection filter). The comb filter is employed in a GA-VSB receiver (see FIG. 1), and disclosed in "Grand Alliance HDTV System Specification," submitted to the ACATS Technical Subgroup, Feb. 1994.
When the NTSC signal is present in the HDTV channel, the NTSC signal acts as interference. In order to reduce this interference, the receiver of the GA-VSB system removes a modulation carrier of the NTSC signal, using an NTSC rejection filter 112, including the comb filter.
FIGS. 2A and 2B show the structure of the comb filter and frequency characteristics thereof, respectively. In a co-channel where the VSB signal and the NTSC signal are simultaneously broadcast, there is a constant carrier frequency offset, approximately 0.89 MHz between the NTSC signal and the VSB signal. The NTSC signal is modulated by a frequency as high as the frequency offset from the standpoint of judging from the baseband region. Most of the energy of the NTSC signal is concentrated in an original DC component, (i.e., a modulation carrier). Accordingly, as shown in FIG. 2A, in a subtracter 132 of the comb filter, a symbol output from a 12 symbol delay 130 is subtracted from an input symbol, to remove the modulation carrier component, which reduces the amount of interference caused by the NTSC signal.
The comb filter, as shown in FIG. 2B, has six frequency notches in an HDTV signal band, that is, a VSB of 6 MHz. Most of the energy of the NTSC signal acting as an interference signal in the HDTV signal is concentrated in a visual carrier (V), a chrominance subcarrier (C), and an aural carrier (A). However, the carriers are each positioned around a frequency notch of the comb filter, so that the energy of the NTSC signal passing through the comb filter is greatly reduced. Accordingly, the comb filter has a simple structure that significantly rejects the NTSC interference signal.
However, although the comb filter proposed by the ATSC standard significantly rejects the NTSC interference signal, it changes the number of output levels of the filtered signal received from an A/D converter 108. That is, due to a partial response characteristic of the comb filter, the number of levels of the signal passing through the comb filter increases from 8 levels {.+-.7, .+-.5, .+-.3, and .+-.1} at the input to 15 levels {.+-.14, .+-.12, .+-.10, .+-.8, .+-.6, .+-.4, .+-.2, and 0} at the output. The operation of the comb filter involves the subtraction of two signals having full gain. Accordingly, a signal of 8 levels increases to 15 levels, and, at the same time, the power of the additive Gaussian noise increases by 3 dB so that the signal to noise ratio (SNR) decreases by 3 dB, while the signal passes through the comb filter.
Also, the design of the TCM (trellis code modulation) decoder 122 must consider the delay caused by the comb filter. Accordingly, when the comb filter is used in the receiver, the TCM decoder 122 must act as an 8-state decoder instead of a 4-state. Therefore, the TCM decoder 122 must include both an 8-state trellis decoder and a 4-state trellis decoder.
As described above, in the case that the comb filter is used for rejecting NTSC interference, an 8-state trellis decoder appropriate for a signal of 15 levels requires a structure more complicated than that of the 4-state trellis decoder. This leads to complex hardware, and degrades the performance of the receiving system due to SNR loss.
However, in order to constrain amplification of white noise to a specified value of 0.3 dB (unlike noise amplification of 3 dB caused by the comb filter) and to minimize NTSC interference, a new GA system using an FIR filter in the transmitter and receiver was proposed. See, International Workshop on HDTV, "A New NTSC Co-Channel Interference Rejection Filter with Coded 6-VSB Modulation for Improved ATV Coverage," by Samir N. Hulyalkar et al., Oct. 8-9, 1996. The GA system proposed in this reference is a new system using a 6-VSB modulation method. The FIR filter is used as a precoder in the transmitter and as an NTSC rejection filter in the receiver, and since the same filter coefficient must be used in both the receiver and the transmitter, the FIR filter cannot apply to the current GA system.